1. Field of the Invention
This invention relates to eddy current probes, and more specifically to a probe with orthogonal solenoid coils with the electromagnetic fields focused through posts.
2. Prior Art
It is known in the art that variations in conductivity and permeability of a material indicate the presence of structural defects such as cracks and corrosion. These variations can be measured by propagating a primary magnetic field into the material to create eddy currents. The eddy currents generated in the material then generate a return magnetic field that is detected by the probe coil. Defects in such materials that decrease the conductivity and disrupt the eddy currents cause the magnitude of the return magnetic field to decrease.
When the material is without flaws, the two magnetic fields are largely out of phase and the fields partially cancel, which reduces the coil voltage. Therefore, the probe coil voltage increases to indicate that the test coil is adjacent a defect. Signature characteristics of the flaw appear as a small modulation of the return magnetic field carrier signal. Thus, the sensitivity of the probe and the ability to sense the signature of the flaw is directly dependent on the magnitude of the incident primary magnetic field. It is therefore advantageous to have a maximum magnetic field strength.
An eddy current probe comprises two solenoid coils wound around a common rectangular wafer base of high permeability material extending beyond the coils at wafer base corners with the base becoming the solenoid core. Posts also of high permeability material depend from a base front at its corners.
A first coil wraps the base around first opposing base sides passing between pairs of the depending corner posts. A second coil wraps the base around second opposing base sides passing between different pairs of the depending corner posts, crossing the first coil orthogonally on the base front and back.
The coils are adapted to connect to alternating electric current such that magnetic fields generated by the two coils have like magnetic poles at base corners diagonally opposed across the base. Connection to electric current varies such that in a first condition the electric currents driving the two coils are in phase producing like magnetic poles at first diagonally opposing base corners. The connection in a second condition is such that the two coils are 180 degrees out of phase producing like magnetic poles instead at second opposing base corners separated circumferentially by the first opposing base corners. Thus, in the first condition, a combined magnetic field of the two solenoids crosses the base diagonally between the first opposing base corners. In the second condition, a second combined magnetic field crosses the base diagonally between the second opposing base corners orthogonal to the direction of the first combined magnetic field.
When the first combined magnetic field penetrates into a material to be tested, eddy currents are generated in the material in a first eddy current direction corresponding to the direction of the first combined magnetic field. The magnitude of eddy currents decreases significantly more for a material flaw not parallel with the eddy current direction than for a parallel flaw. Thus, by alternating between orthogonal first and second combined magnetic fields a flaw that might be less detectable as a result of a combined magnetic field in a first direction might be better detected by the other combined magnetic field in a direction orthogonal to the first.
The magnetic field generated by eddy currents generally in the material constitute a carrier signal that affects voltage in the orthogonal coils that is not dependent on magnetic field direction. Small changes in the eddy current magnetic field are modulations on the carrier signal. Subtracting the voltage signals in the respective coils therefore largely removes the carrier signal leaving mostly the change in eddy current signal due to a presence of a material anomaly.
The magnetic field strength decreases with distance from the solenoid. Therefore, it is advantageous to have the field source as close to the material to be tested as possible. The wafer base corners from which the combined magnetic fields emanate necessarily are separated from the test material at least by the thickness of the coils with the resulting decrease in magnetic field strength at the material. To direct the combined magnetic field from the solenoid to the test material with minimum loss of field strength, posts are located on the base corners from where the combined magnetic fields emerge from the two orthogonal coils. The combined magnetic fields are largely conducted through the posts and out post ends. The post ends are of length greater than the extent of the coils on the base and of sufficient length to place the post ends proximate the material to be tested, ideally located in near proximity to the material surface to minimize loss of field strength.
Post ends have a curvature matching a material to be tested to improve coupling of the magnetic field from the post into the test material. That is, for a flat test material, the post ends are flat; for a curved material such as a tube tested from its inside, the post ends are concave with a radius matching the inner radius of the test tube. Without a matching post end curvature, only an edge of the post is in near contact with the curved test material, the remaining post end being separated from the test material resulting in xe2x80x9clift off,xe2x80x9d a reduction of eddy current signal strength due to poor or inconsistent coupling of the magnetic field into the test material, a common cause of which is separation of the probe drive coils that excite eddy currents in the test material from the surface of the test material. Varying distance of the solenoid coil from the test material as the probe translates along the material also changes magnetic field strength coupled into the test material and hence the magnitude of eddy currents generated by the magnetic field. These changes in the eddy current signal while actually caused only by varying lift off may be interpreted as a material flaw. Therefore it is advantageous to have the post ends consistently and uniformly riding on the test material surface or at least a close measured distance from the material with post end curvature matching test material curvature.